Channel types

When declaring variables of channel type, the most common instances are like this (T is any valid type):

var v chan T

But you may also see examples as follows :

var v <-chan T


var v chan<- T

What the hell are the differences among these 3 definitions? The distinctions are here:

(1) chan T: The channel can receive and send T type data;
(2) <-chan T: The channel is read-only, which means you can only receive T type data from this channel;
(2) chan<- T: The channel is write-only, which means you can only send T type data to this channel.

The mnemonics is correlating them with channel operations:

v := <-ch  // Receive from ch, and assign value to v.
ch <- v    // Send v to channel ch.

<-chan T is similar to v := <-ch, so it is a receive-only channel, and it is the same as chan<- T and ch <- v.

Restricting a channel type (read-only or write-only) can let compiler do strict checks for you. For example:

package main

func f() (<-chan int) {
    ch := make(chan int)
    return ch

func main()  {
    r := f()
    r <- 1

The compilation generates following errors:

invalid operation: r <- 1 (send to receive-only type <-chan int)

Furthermore, the <- operator associates with the leftmost chan possible, i.e., chan<- chan int and chan (<-chan int) aren't equal: the previous is same as chan<- (chan int), which defines a write-only channel whose data type is a channel who can receive and send int data; while chan (<-chan int) defines a write-and-read channel whose data type is a channel who can only receive int data.

Channel types;
How to understand "<-chan" in declaration?.

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